(1) Field of the Invention
The present invention relates to telephone subscriber supervisory circuits and more particularly to a circuit for connecting and disconnecting a ringing voltage generator to a telephone subscriber's loop circuit prior to application and after termination of ringing voltage, to said loop circuit.
(2) Description of the Prior Art
When a telephone subscriber's loop circuit is switched between a DC transmission source and a ringing voltage generator, substantial transients are typically generated by the interruption of these circuits. These transients are particularly harmful to relay contacts and digital networks.
This problem has traditionally been solved by resistor/capacitor networks, other types of contact transient suppression circuits, mercury wetted relays, or sequential relay contact operation. A contemporary approach has been to sense the absence of ringing voltage and to operate the relay performing the switching (ring relay) at the instant the ringing voltage cycle crosses zero volts. However, this approach requires precise operation of the ring relay contacts in order to switch the ringing voltage generator at the opportune time. These prior art methods all incur substantial penalty in the form of cost, size, and/or limitations on performance.
Accordingly it is an object of the present invention to provide a novel low cost, maintenance free, highly reliable technique of switching the telephone subscriber's loop between a DC transmission source and a ringing voltage generator.